The administration of intravenous medical fluids to a patient is well known in the art. Typically, a solution such as saline, glucose or electrolyte contained in a glass or flexible container is fed into a patient's venous system through a conduit such as a polyvinyl chloride (PVC) intravenous (IV) tube which is accessed to the patient by a catheter. Many times, the fluid is infused under the forces of gravity, and the rate of flow is controlled by a roller clamp which is adjusted to restrict the flow lumen of the IV tube until the desired flow rate is obtained.
Flow from the container to the patient also is known to be regulated by means other than a roller clamp. It is becoming more and more common to use an electronically controlled infusion pump. Such pumps include, for example, peristaltic-type pumps and valve-type pumps. Peristaltic-type pumps typically include an array of cams angularly spaced from each other which drive cam followers connected to pressure fingers. These elements cooperate to impart a linear wave motion on the pressure fingers. This linear wave motion is used to apply force to the IV tube, which imparts the motion to the fluid in the IV tube thereby propelling the fluid. An alternative type of peristaltic pump employs a plurality of roller members which roll over the IV tube to impart the motion to the fluid in the IV tube. Infusion pumps also employ pumping chambers having upstream and downstream valves to sequentially impart the propulsion to the fluid. Such valve-type pumps typically require the use of a specialized pumping cassette chamber, which is contained on a dedicated IV tube between the patient and the source of fluid.
A particular concern in infusing a patient with medical fluid is that air will be introduced into the IV tube, which can then be infused into the patient's venous system. Prior art pumps have employed air detectors designed to detect the presence of air in the IV tube. Such air detectors, however, have had a relatively small dynamic range for testing, which has made such testing sensitive to the type of IV tube utilized and the temperature of the pump environment.
What is needed is an air sensor for a medical infusion pump which has a large dynamic range for testing, so that positive delineation is possible under a wide range of IV tubes and reasonable temperature extremes. It would be advantageous for such a sensor not only to sense air in an IV tube, but also to discriminate between an empty IV tube and no IV tube. It would be further advantageous for such an air sensor to have a miniature size capable of being employed while minimally adding size and weight to the pump housing. It would be further advantageous for such an air sensor to use low power consumption so that, when the pump is operating on auxiliary battery power, battery life is extended.